Some power MOSFETS designs include dielectric lined polysilicon filled trenches as their gates. In this power MOSFET structure, the gate is buried in a trench etched in the semiconductor, such as a substrate comprising silicon. This arrangement results in a vertical channel.
Other power MOSFETS designs use dielectric lined polysilicon filled trenches as field plates and provide a substantially planar FET structure, where the trench polysilicon is connected to the source (and generally also the body). For example, these MOSFETs have a gate structure and a vertical drain drift region between polysilicon filled trenches configured to act as field plates sometimes referred to as “RESURF trenches”. For purposes of this patent application, the term “RESURF” is understood to refer to a material which reduces an electric field in an adjacent semiconductor region. A RESURF region may be for example a semiconductor region with an opposite conductivity type from the adjacent semiconductor region. RESURF structures are described in Appels, et. al., “Thin Layer High Voltage Devices” Philips J, Res. 35 1-13, 1980.
The RESURF trenches contain a dielectric liner and are generally filled with doped polysilicon. In the active region for n-MOSFET, the RESURF trenches (hereafter “active area trenches”) are polysilicon field plates which are electrically coupled to the source electrode. In the case of an n-MOSFET, there is a p-body region within an n-drift region on a substrate, where n-type dopants are in the source regions formed in the p-body region. The drain for the n-MOSFET can be a vertical drain drift region that uses the entire n-drift region below the p-body region that has a drain contact on the bottom of the substrate, which can be an n+ substrate.
Related U.S. application Ser. No. 13/744,097 to Kocon et al. hereafter “the '097 application” where Kocon is one of the inventors of this application as well, discloses the MOS device described above having a substantially planar gate structure on a drift region of a first conductivity type and a body region of a second conductivity type formed in the drift region, having sources for n-MOSFETs formed in the body region. A vertical drain drift region is positioned between active area trenches.
A contact metal stack makes electrical contact with a source region for the MOSFET at lateral sides of the contact structure, makes electrical contact with a body contact region at a bottom surface of contact structure, and makes electrical contact to the polysilicon field plates in the active area trenches. Another RESURF trench which is referred to as a “termination trench” is at a perimeter around the active area trenches.